The present invention relates in general to high pressure direct injection systems for internal combustion engines and in particular to high pressure piston pumps used in such systems.
In high pressure direct injection systems (gasoline or diesel), high pressure (HP) pumps with fixed fluid displacement are typically used. The fluid delivered by the pump is dependent only on the engine rpm and not on the amount of fuel injected into the combustion chambers. The HP pumps are usually oversized so that under all circumstances there is enough fuel flow. Therefore, under light engine load conditions, the pump delivers too much fuel, because only a small amount of the delivered fuel is injected. Similarly, under light engine load conditions, the engine power used to drive the pump is unnecessarily large, resulting in a loss of fuel efficiency.
For future automotive applications, reduced power consumption of the BP pump will be of higher importance. This is particularly so when considering that the power consumption of the HP pump of the future may be two to four times higher than the present power consumption. For example, the fuel rail pressure may be doubled to 250 bar or the HP pump size may be increased for high displacement engine applications (V6 or V8 engines). These applications may need four times more fuel flow than at present.
Variable flow control HP pumps are necessary to reduce parasitic losses attributable to the HP pump and thereby increase engine efficiency. Also, a variable flow HP pump can deliver fast and safe engine starts, that is, fast fuel rail pressurization, without the parasitic pump losses after engine start. Additional advantages of variable flow HP pumps include less fuel heatup, downsizing of related components and possible elimination of some components, for example, the HP fuel regulator.
The advantages of variable HP pump flow are even more apparent when one realizes that the HP pump displacement is determined only by cold engine start requirements. Therefore, after cold engine starts using a high pressure start strategy, the HP pump fuel delivery is typically three times greater than needed for full load engine conditions. Even in the case of high pressure direct injection engines with a low pressure start strategy, a variable pump flow is desirable because the engine runs only a small part of its operation time at wide open throttle (WOT). That is, the high fuel flow delivery from the HP pump is needed only a few times during engine operation.
One proposal for a variable flow pump is a pump with infinitely variable delivery control. However, such a pump is very complicated. An alleged advantage of the infinitely variable delivery control pump is the elimination of the regulator valve. However, from a safety standpoint, if the regulator valve is eliminated, one would need a second safety valve for redundancy. Therefore, elimination of the regulator would not actually be a cost saving. In the present invention, the engine electronic control unit simply provides an on/off signal to a deactivation solenoid.